I like ages.
It's a nice way for enforcing cross-fields requirements, without making complex (and somewhat arbitrary) tech requirements.
Think about what a "plane" tech tree could look like:
helix plane -> jet plane -> supersonic jetplane -> modern supersonic jetplane -> drone
Each tech level takes more than it looks, from other fields that have nothing to do with planes:
* To get to jet planes, you requires jet engines. Okay, that is the next improvement and iit looks like that's enough.
* To get to the supersonic jet plane, you need pure plane improvement (you need delta wings for exemple), but also specific motorization improvement (engines that can accept supersonic input flow), as well as material improvement (first planes just dissassembled in the sky while passing the wall of sound) ; a bit more complex than just "plane tech".
* To get to a modern jet plane, you have to throw in sensors (that tells the pilot the state of the plane), radars, electronic commands, computer assisted navigation and whatever you want to throw in. None of which techs remotely come from "plane" background! The improvement now comes from taking from other technologies (many) and putting them into planes. But there is no discussion that a modern jet fighter would blow apart a WWII jet fighter! So that's indeed a new tech.
* To get to drones, you have to throw the pilot out and replace him with more sensors and commands, working remotely. That requires complex and high speed secure transmissions, possibly satellites... Once again it's done by integrating new technology thats outside the field of the "plane" itself into the concept of the plane.
That's what ages are all about: you can't go further because your research base is too tiny.
Indeed, another acceptable way to get rid of them is a complex formula that would basically use exceedingly expensive research costs, then discounts based on the number of lower techs of whatever field you master.
Game wise, the idea is also to prevent the possibility of winning easily by mounting super-powerful weapons on throw away cheap ships: too simple strategies such as climbing too fast into the tech tree should be prevented or the game might risk becoming not fun. Civilization imposes it through multi-tech dependencies. GC3 does the same using the concept of ages.
For my part, I even think that ages go by too fast...